McChooky
Free Ranging
After a mysterious disease killed 2 hens I fenced my chickens out of my compost piles and only recently started composting again(my chickens are fenced out of both)Wyorp mentioned if your hens could have gotten into something mouldy and you mentioned they have access to the compost. I used to keep my compost where my hens could access it and then after reading up on botulism I moved it… scary stuff. It can cause sudden onset of symptoms and death very quickly. Not saying that’s what’s happening for sure in your case but it’s something to seriously consider.